The Moon is a balloon...

 From:  Michael Gibson
4518.2 In reply to 4518.1 
Hi Jean-Paul, maybe something along these lines - starting with some curves arranged liek this:

The outside ring gets extruded to make a solid, then use Boolean difference to cut that solid by the other 2 inner pieces, so you've got 3 solids like so:

Put a big fillet on the outside edge:

A little bit less fillet radius on the inside parts:

Maybe use the G2 Blend fillet shape rather than a circular fillet cross-section, and another possibility might be to slice off the front face with a slightly rounded surface to make it look less totally flat in front, see for example this previous post for an example of that kind of thing:

It's also possible that you may want to make some kind of tubular sweep for the outermost one instead of a filleted block but maybe the inside pieces would be good to do with fillets on extruded pieces.

- Michael